1. Field of the Invention
The present invention relates to an 1xc3x972 thermo-optic switch, and more particularly to a thermo-optical switch, which has an excellent optical crosstalk without a loss of the driving voltage while reducing a difficulty in fabricating the switch.
2. Description of the Prior Art
A conventional Y-branch type 1xc3x972 digital optical waveguide switch is more advantageous in comparison with a Mach-Zehnder modulator, in that the Y-branch type 1xc3x972 digital optical waveguide switch has a digital switching characteristic independent to the wavelength and the polarization, since the Y-branch type 1xc3x972 digital optical waveguide switch utilizes a mode evolution, not a mode interference, which the Mach-Zehnder modulator utilizes.
In contrast, the digital optical switch is disadvantageous in that it is very difficult to fabricate elements of the digital optical switch, since the digital optical switch, which utilizes the mode evolution characteristic, can reveal an excellent crosstalk without any loss in a propagating transmitting mode, only when the branching angle at a Y-branch region, at which the waveguide is divided into waveguides of a fundamental mode, namely the 0th mode, and a higher-order mode, namely the 1st mode, is very small, generally about 0.1xc2x0.
FIG. 1 illustrates a conventional 1xc3x972 digital thermo-optic switch of Y-branch type using mode evolution. In FIG. 1, parameters are used as follows: nclad means a refractive index of a cladding layer; ncore means a refractive index of a core layer; Ew means a width of a heater; w means a width of a waveguide; xcex1 means a Y-branch angle; P means an electric power applied from the exterior; Pin means an optical power of input light; P3 means an optical power transmitting out of an upper arm 3 of a Y-branch; and P4 means an optical power transmitting out of a lower arm 4 of the Y-branch.
Referring to FIG. 1, the conventional Y-branch thermo-optic switch has a waveguide construction, which comprises an input section 1 of a single mode, a Y-branch section 2 with a branching angle xcex1, and an output section consisting of upper and lower arms 3 and 4 respectively of a single mode.
Upper and lower heaters 5 and 6, respectively for controlling the path of the propagating light, are respectively spaced apart from each of the arms with a distance xcex4, by which the temperature difference between both arms of the Y-branch can be maximized. In this case, the distance xcex4 is so optimized as to maximize the difference between refractive indices of both arms when an external electric power is applied to one of the arms. The branching angle xcex1 of the Y-branch section is designed in such a manner as to maximize the crosstalk at the final output ports 3 and 4 of the thermo-optic switch while minimizing the optical loss at the Y-branch.
Mode evolution is a phenomenon, in which a propagating light undergoes a conversion from one waveguide mode into another without any optical power loss when there is a gradual change of the refractive index in the waveguide structure along the propagating direction of the waveguide. In order to generate the mode evolution at the branching region of the Y-branch type waveguide, the branch angle xcex1 should satisfy the following Equation 1.
xcex1 less than xcex94xcex2/"khgr"xe2x80x83xe2x80x83Equation 1 
In equation 1, xcex94xcex2 is a propagation constant difference between the 0th mode and the 1st mode at the branching region, and "khgr" is attenuation constant at the Y-branch cladding region.
When electric power is applied to the upper heater 5, the temperature of the upper arm 3 increases to be larger than that of the lower arm 4, so that the refractive index of the upper arm 3 decreases to be smaller than that of the lower arm 4 (in the case where it is made from polymer material) due to the thermo-optic effect. In this case, among the light beams having transmitted through the Y-branch, the light in the 0th order mode undergoes a mode conversion into the 0th mode of the lower arm 4 having a relatively higher refractive index with no applied electric power, while the 1st order mode is converted into the 0th order mode of the upper arm 3 having a relatively lower refractive index. In this case, when all the input light beams are in the 0th order waveguide mode, they progress into the lower arm 4. However, usually at the Y-branch region is excited a small amount of the input light in the 1st order waveguide mode, which then progresses into the lower arm 3 to cause a crosstalk. In order to obtain a superior crosstalk, which is one of the most important characteristics in an optical switch, in other words, in order to maximize the optical output ratio between the power-applied arm 3 and the other arm 4, the amount of light guided into the waveguide output port 4, to which no electric power is applied, has to be maximized while the amount of light guided into the other waveguide output port 3 has to be minimized. The optical crosstalk X means a difference between output optical powers of the two arms and is defined as the following equation 2 when the input light is switched into the lower arm 4.
x=10*Log(P3/P4)xe2x80x83xe2x80x83Equation 2 
On the contrary, when an external electric power is applied to the lower heater 6, the input light is outputted through the upper waveguide 3. Since the optical switch using the mode evolution as described above generally requires a very small branch angle xcex1 nearly of 0.1xc2x0, it has been very difficult to actually fabricate the conventional optical switch using the mode evolution.
Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a new structure of a digital thermo-optic switch that has an excellent crosstalk without increasing the driving voltage and also can reduce the difficulty in fabrication process by increasing the Y-branch angle.
In order to accomplish this object, there is provided a thermo-optic switch comprising: a basic Y-branch optical switch having upper and lower arms; at least one variable thermo-optic attenuator connected to at least one of the upper and lower arms of the basic Y-branch optical switch, the variable thermo-optic attenuator utilizing a higher-order mode generator; and at least one attenuator heater connected to the variable thermo-optic attenuator, so as to heat the variable thermo-optic attenuator, thereby controlling a refractive index of the variable thermo-optic attenuator.
The present invention integrates variable thermo-optic attenuators utilizing a higher-order mode generator that requires relatively low driving voltage with a conventional Y-branch type thermo-optic switch. As a result, even when the crosstalk is not good at the conventional Y-branch type thermo-optic switch output ports, the final crosstalk at the inventive optical switch output ports is excellent by removing the leaked light at the optical attenuator region.
Also, since the crosstalk of the conventional Y-branch type thermo-optic switch itself is not critical, the Y-branch angle can be increased and by increasing the branch angle the electrode length of a conventional Y-branch type optical switch can be decreased. Since this decreased length of the electrode is about same size as the newly added length of electrode for variable thermo-optic attenuators, the driving voltage of the present invention does not increase as a whole. In addition to this, by increasing the branch angle, the difficulty of fabrication process decreases and the size of the whole device decreases.
Improvement of the crosstalk characteristics while reducing the difficulty in fabrication process of a conventional Y-branch type digital optical switch can be achieved by integrating variable optical attenuators at the end of both arms of output ports to remove the residual light which leaked through the off-state arm at the Y-branch of a conventional 1xc3x972 optical switch. This general structure, however, has disadvantage of requiring very large driving voltage because it needs to drive optical attenuators in addition to the conventional optical switch.